The invention relates to a process for generation of engraving data, especially for an engraving cylinder, based on a vectorized printer""s copy and a screen stipulation. Furthermore the invention relates to a device for engraving of a engraving cylinder according to the engraving data generated using the process.
Electro-mechanical engraving of printing forms has been possible at latest since the 1970s with high precision and reliability. Here it is known (compare for example EP 0 056 829 B1) that in four color printing the screens of the different colors must be matched to one another such that no visible color shifts occur (color drift).
In the production of engraving data (i.e the data with which the engraving machines are triggered to machine the engraving cylinder) from a digitally stored printer""s copy, conventionally first a high definition bit map is produced to undertake reduction to the screen density stipulated by the client.
It is apparent that in the transition from a high resolution bit map to the stipulated screen density a loss of reproduction accuracy must be tolerated. In practice this often leads to fine line patterns and fonts becoming blurred or irregular in the sense of a Moire effect. In multicolor print products therefore on the line boundaries unwanted color effects can arise because based on the different color screens different line boundaries cannot be avoided.
The object of the invention is to devise a process of the initially mentioned type which ensures the highest possible image quality (xe2x80x9csharpnessxe2x80x9d, xe2x80x9ccolor precisionxe2x80x9d) even in finely structured printer""s copies.
The object is achieved by the features of claim 1. As claimed in the invention not simply the stipulated print density (screen fineness) is used (as is actually expected or required by the client). Rather around a stipulated print density a variation range is defined and then within this range depending on the geometry of the elements contained in the actual printer""s copy an optimally matched screen is determined.
The invention is not concerned with simply choosing the finest possible screen within the (pre-)defined variation range. Depending on the type of geometry (for example, periodicity of a line pattern) it can be that a slightly coarser screen coincides with the geometry (for example periodicity) and thus allows a image of maximum sharpness. Here it is important that the engraving in principle can produce any screen (for example 70.65 or 63.88) and accepts not only discrete tabular values (for example 65, 70, 90). Furthermore it should be watched that with the process as claimed in the invention the best possible use is made of the initial data which are not definition-limited and thus a high definition bit map is abandoned overall.
The variation range is an acceptably chosen parameter of the process as claimed in the invention. It is typically a maximum 1/3 of the stipulated screen definition and can for example be 20% or less. It need not be symmetrically (for example +/xe2x88x9210%) arranged with respect to the stipulated screen value, but can also be shifted against fine values (for example +12%/xe2x88x928% screen stipulation).
In the first step, in the printer""s copy line elements are determined (this can be done based on vectorized data with relatively simple search commands). Then for them, within the variation range, a so-called working screen which is matched to the geometry (for example width, distance and/or periodicity) of the lines is determined. Depending on the result the working screen can be accepted directly as the engraving screen or further varied and modified. The objective of matching is to bring the half-tone dots (screen cells) and the line (element) boundaries into congruence.
In a following step the font elements of the printer""s copy are identified and based on the existing font hinting data are adapted to the working screen. I.e. the font elements (letters with a stipulated font type) are shifted within the boundaries defined by the font hinting data, stretched or flattened in order to bring the horizontal and vertical sections into congruence with the working screen.
In multicolor printing stipulated screen sets are conventionally used. If therefore the screen for one of the colors is adapted to the elements of the printer""s copy according to the described process steps, this also yields the working screen of the other colors (compare for example EP 0 056 829). According to the basic idea of the invention however they should also be in agreement with the line and font elements. But generally this is not the case. For this reason the screens of the other colors are also matched in the described manner to the elements of the printer""s copy. It is not precluded that for one color an optimally matched screen can be found, but that the pertinent other color screens cannot be sufficiently (i.e. within the allowable limits) stretched, flattened, enlarged or reduced to obtain an acceptable result. Then it is necessary to choose either the aforementioned xe2x80x9coptimally matchedxe2x80x9d color screen only xe2x80x9csuboptimally matchedxe2x80x9d to improve the overall result, so that for the other color screens the desired matching accuracy is possible, or the printer""s copy is rejected (for purposes of revision by the graphics specialist).
To further improve printing results the working screen can be matched to the dimensions of the copies (for example the individual label). Also here stretching or flattening of the screen geometry is used.
Finally it can also be provided that between the individual copies in the axial direction of the engraving cylinder (=advance direction of the engraving head) small intervals are inserted. Here xe2x80x9csmallxe2x80x9d means distances which are smaller than the distance between the half-tone dots. They should also lie within the aforementioned variation range of the screens. In this way it is ensured that the copies have not only essentially the same quality, but are completely identical. That is, in each copy possible residual errors are invisible at exactly the same site and in the same way.
Within the framework of the invention it is also possible to vary between the different screens in the advance direction. The purpose of this measure can be for example to achieve an optimum print quality which is matched to each individual copy when entirely different copies (for example, different labels attached to a bottle) are engraved on a cylinder.
The input data can be accommodated in a single data block (file) or also in several separately. In the former case engraving takes place in one pass without interruption. In the latter case the engraving head can (or must) be briefly stopped between the individual data blocks to adjust the control parameters for the modified screens.
In itself the process as claimed in the invention is independent of which engraving technique is used. But it is used mainly for machines which work with laser technology and electromechanically (the latter have an oscillating graving tool for engraving). One such machine is made and can be triggered such that the line intervals in the feed direction and the dot distances (i.e. the distances of the screen cells) are essentially freely adjustable within the lines.
Other advantageous embodiments and combinations of features of the invention follow from the following detailed description and the totality of claims.